Scheme.forth.jl: Scheme in Forth in Julia

scheme.forth.jl

A hobby Scheme interpreter for FORTH 83. Specifically it is targeted at forth.jl which is an implementation of FORTH on top of Julia , hence the name. It began life as a fairly direct port of Peter Micheaux's Bootstrap Scheme (as described in this wonderful series of blog posts ) from C to forth, but also includes:

call-with-current-continuation

Running the interpreter

To run this Scheme interpreter, first open Julia (forth.jl requires version >=1.0 ) from the src directory contained in this repository. If you've not done so already, install forth.jl using the Julia package manager (accessed using the ] key):

(v1.0) pkg> add https://github.com/tgvaughan/forth.jl

Then, import and run the Forth system:

julia> import forth
julia> forth.run()
Welcome to forth.jl!

Once Forth is running, execute the Scheme source and fire up the REPL using the following commands:

include scheme.4th  ok
scheme repl
Welcome to scheme.forth.jl!
Use Ctrl-D to exit.

>

At this point you can start entering Scheme commands. For example,

> (define (factorial n)
    (if (= n 0)
      1
      (* n (factorial (- n 1)))))
; ok
> (factorial 5)
; 120

Metacircular Evaluator

Of course, one of the things you can do in Scheme (or of course any programming language, this is the fundamental thing) is implement an interpreter for another programming language. The examples directory in this repository contains a verbatim copy of the source for the "metacircular" scheme interpreter from SICP. To load it, use the following command:

> (load "../examples/metacirc.scm")
; ok

Be prepared to wait a couple of minutes. When the interpreter finally loads, enter the following command to run it:

> (driver-loop)

You'll then be greeted by the following prompt:

;; M-Eval input:

At this point you can start entering Scheme commands... but be prepared to wait a while for each result. After all, when evaluating commands in the MCE you are running a program in a Scheme interpreter running inside another Scheme interpreter which is itself running on a Forth system that is implemented atop a virtual register machine running in the Julia numerical computing environment. That's four levels of abstraction more than a native Julia program experiences , so some delay is to be expected!

For instance, the following example from SICP defines and demonstrates a recursive list append procedure:

(define (append x y)
   (if (null? x)
       y
       (cons (car x)
             (append (cdr x) y))))

 ;;; M-Eval value:
 ok

 ;;; M-Eval input:
 (append '(a b c) '(d e f))

 ;;; M-Eval value:
 (a b c d e f)

You may have to wait a minute or so for the final result to be printed.

License

This software is free (as in freedom) and is distributed under the terms of version 3 of the GNU General Public License. A copy of this license is included in this repository in the file COPYING.